docs: add README, rename font license file

README.md

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+# previewBOT
+
+A Discord bot written in Rust for sending file previews of GitHub URLs and for juxtaposing images.
+
+## Invite
+
+You can invite the official instance of this bot to your server.
+
+> https://previewbot.kneemund.de/invite
+
+## Details
+
+Provides an HTTP API for detailed views of juxtaposed images on the web. The source code of the website used for the official instance of this bot is available [here](https://github.com/Kneemund/juxtapose).
+
+A container image can be built by using the provided `Dockerfile`. It supports fast multi-architecture builds for amd64, aarch64 and arm/v7 using cross compilation instead of emulation. The produced binaries are fully statically-linked using `musl` and `mold`. As such, the image is derived from the empty `scratch` base image and only contains the binary.
+
+Running the bot with the `--reload-commands` argument will register all slash commands after connecting to the Discord API. This is only necessary on new accounts or after changes to the structure of slash commands.
+
+## Enviornment Variables
+
+All environment variables without a default value must be specified, otherwise the application will panic (usually during startup). If a `.env` file exists within the working directory, the location of the file is logged and it will be parsed and loaded while keeping the values of already existing environment variables.
+
+| Name                | Default Value        | Description                                                                                                                                                            |
+| ------------------- | -------------------- | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
+| BOT_TOKEN           | NONE                 | Secret token for the bot account created in the Discord Developer Portal.                                                                                              |
+| BLAKE3_KEY_MATERIAL | NONE                 | Master secret key for deriving other keys using the BLAKE3 KDF, e.g. the key for creating and validating the HMAC in Juxtapose URLs.                                   |
+| JUXTAPOSE_BASE_URL  | `http://localhost`   | Base URL used for viewing juxtaposed images, used for generating URLs for the "Open" button.                                                                           |
+| REDIS_URL           | `redis://127.0.0.1/` | URL used for connecting to Redis. Can be either a TCP connection (`redis://` or `rediss://`), or an IPC/UNIX connection (`redis+unix://`).                             |
+| PORT                | NONE                 | Port number that the HTTP API runs on.                                                                                                                                 |
+| CORS_ORIGIN         | `*`                  | Allowed origin domains for CORS. Allows all domains by default, but is highly recommended to be set to a specific domain in production (typically JUXTAPOSE_BASE_URL). |